Shulou(Shulou.com)11/24 Report--

In the 1820s, British physicist James Bradley (James Bradley,1693~1762) explained the cause of the aberration angle and obtained the optical speed value from it. For details, please refer to the previous article "Relativity Series: classical Optical aberration".

It is worth noting that Bradley did not adopt the wave theory of light to explain the aberration of light. He believes that the particles of light say that they do not need an "ether". Therefore, the ether in his eyes is just an alias for absolute space.

In Bradley's view, light is like some kind of ballistic particle flow, and since raindrops are tilted backward relative to pedestrians, moving light particles have a similar phenomenon. Therefore, although he adheres to the "theory of particles of light", it does not affect his explanation of the aberration of light.

Not only Bradley, but in the 18th century and even the early 19th century, most people in the mainstream scientific community believed in the theory of particles of light. The particle of light is so attractive, what on earth does it say?

According to the theory of light particles, light is a special kind of tiny particles. In his optics book Opticks, Newton called it corpuscule, so the particles of light are called corpuscular theory of light.

Just like the associated meaning of "blood cell" in corpuscule, light particles are not perfectly smooth rigid spheres, they have irregular surfaces. For this reason, Newton wanted it to provide an explanation for birefringence, which seemed to be much more advanced than Huygens thought that light was a longitudinal wave and could not explain polarization.

In addition, Newton believed that the particles were colored, which made the experiment of dividing white light into multiple colors by a prism seem reasonable.

But none of this is the most important thing. Most importantly, Newton's particle theory can better explain the three basic laws of geometrical optics: the law of linear propagation, the law of reflection and the law of refraction, which have been fully tested and generally accepted by experiments before Newton's time.

Unlike proponents of the etheric wave theory, Newton believed that the speed of light was not given by the etheric. In his view, the ether is almost a superfluous concept. Light particles move in space, and if there is a transparent substance in space-the medium, the light particles also pass through the medium at the same time.

The particle theory holds that in addition to the relative speed of the luminous object and the observer, there are two factors that determine the speed of light.

The first factor is gravity.

A ball thrown flat, it is affected by gravity, the speed in the vertical direction is increasing, while the ball thrown vertically upward, in the process of upward movement, its speed will decrease.

Similarly, when light is emitted from a star into space, it slows down because of gravity; conversely, when light is sent from space to earth, gravity causes light to speed up.

The second factor is the density of the medium, that is, the refractive index.

Contrary to the wave theory, Newton believed that when light hits denser matter, the speed of light increases and vice versa. Assuming that the refractive index of the two media is and respectively, the velocity and satisfaction relationship of light in the two media mentioned above, Newton's particle theory can explain the three major experimental laws of geometrical optics. The following is illustrated by the typical law of refraction.

The law of refraction was first discovered by British physicist Thomas Thomas Harriot,1560~1621 in 1602. In 1621, Willebrord Snellius,1580~1626, a Dutch physicist, discovered this law again in his experiment and got its name Snell's law.

When the law of refraction was discovered, Newton, born in 1642, did not know where to lick the pig trough. But based on the particle theory invented by Newton, we can deduce this law.

According to Newton, if the speed of light in a vacuum is zero, the speed of light in a medium with a refractive index is

As shown in the figure above, for a medium with refractive index of light incident from vacuum, because the velocity component parallel to the interface is constant, while the vertical component is larger, the relationship between the components of light is based on the triangular relationship. obviously, when light is incident from a medium with a refractive index to a medium with a refractive index, a general conclusion can be drawn that this is Snell's law.

That's all I have to say about particles, let's go back to Bradley's explanation of aberration.

02. The problem of classical aberration theory. Look at Bradley's aberration formula.

What is in the formula is the speed of the light particles relative to the observed star, while the speed of the earth relative to the star.

Bradley will take it as the speed of the earth's revolution, and, in his view, the speed of light particles relative to the star is also regarded as constant.

Obviously, this explanation is logically problematic.

On the one hand, the premise that the speed of the earth can be taken as the revolution speed of the earth is that the relative speed of the earth is the same relative to the sun and any other star.

But obviously, there is no reason for stars to stand still relative to the sun. In other words, different stars should have different speeds relative to the earth!

On the other hand, the premise that the speed of light can be treated as constant is that the speed of light is not affected by stars.

But since light is a particle, for different stars, light is like a marble ejected from a star, and its speed should be affected by the star.

The first effect comes from the speed of the star, which seems to be taken for granted. As a sprinkler for spraying light particles, the speed of its movement will certainly affect the speed at which light particles are emitted.

But at the time, the effect, if any, could not be known, because the actual speed of the star was unknown, and it was generally believed that the star was stationary in absolute space, otherwise it would not have been called a fixed star-- star.

The second effect comes from the mass of stars. Newton is the creator of the theory of universal gravitation. "gravitation" means the "absolute gravitation" between any matter, so the light particles emitted from stars are also gravitated by stars when they are far away from the star.

Stars with different masses have different gravitation, and according to Newton's particles, the speed of light particles from different stars must be different.

Now we know that, according to the theory of relativity, the speed of light is constant, so neither of these effects exists. However, if we want to understand the theory of relativity well, we should follow the real historical process to explore it. Only in this way can we understand the logical relationship between various experiments in the development of theory and the inevitability of theoretical choice.

So, how did those smart brains in history solve these problems and go through a difficult and tortuous process of exploration and finally lead to the correct theory?

03. Fran ç ois Arago,1786~1853 's study A French physicist named Francois Arago appeared.

Most people may not have heard of Arago, but in fact, he is a rare top genius and master in the history of physics. The famous "Poisson spot" was actually discovered by him in the experiment, but it was named to satirize Poisson's original mockery of wave theory. In addition, the famous Arago disk experiment in electromagnetics also got its name from him.

The young Arago believes that for different stars, due to the different speed and mass of stars, as mentioned earlier, the speed of light will be different, resulting in different aberration angles. However, there are no different aberration angles in a large number of optical aberration observations.

One natural explanation for this is that although it is different, because the ratio is much larger, the change of the aberration angle caused by the change of the ratio is very small. For example, the difference of the ratio 20 leads to the change of the aberration angle of only 1 ". This is an accuracy that can not be achieved by instruments at that time.

Arago thought that when light with different light speeds is refracted by the same medium, although the incident angle is the same, the refraction angle will be different. Therefore, if the speed of light of different stars is different, this can be confirmed by observing different refractive angles.

You may ask: how can there be different refraction angles for light with different speed of light and the same incident angle? Because according to the law of refraction, when the incident angle is the same, the refraction angle only depends on the ratio of the refractive index of the medium on both sides of the interface! Isn't that a contradiction?

But Arago found this contradiction in theory, and he reasoned based on Newton's theory of particles. Let's take a look at the logical reasoning behind this.

At present, there are two beams of light with different velocities, the velocities are and, the media with the refractive index enter the refractive index at the same incident angle as the medium, and the velocities become sum respectively, as shown in the following figure.

Because the incident angle is the same,

Because and and, so the same reason, so the right side of the above formula is not equal, which shows that the refraction angle of the two is different!

However, Arago did not find different speeds of light in the experiment, and the law of refraction appeared to be intact.

But Arago was undaunted, and he came up with a new idea.

He believes that the speed of light that satisfies the law of refraction is the speed of light relative to the medium. Since the prism in the experiment moves with the earth, the speed of light that really works is the speed of light relative to the earth.

If the speed of light is constant for absolute space, but the earth is moving, then according to Galileo's principle of relativity, the speed of light should be different in different directions on the earth. In particular, this difference is most obvious when the direction of the speed of light is the same as and opposite to that of the earth.

Obviously, this difference is the evidence that the earth moves in absolute space.

The experiment he completed in 1810 was simple. He used a prism with an inclination of 24 degrees to cover half of the telescope, so that he could see both the direction of starlight and the angle of deviation refracted by the prism, thus deducing the corresponding refraction angle.

Unfortunately, his experiment failed again, and he did not find the effect of the earth's motion on refraction.

Although the results of Thomas Young's double-slit interference experiment on light were published in 1807, the influence may not be enough. Arago still believed in the theory of particles in 1810. He insists that the speed of light from different light sources should be different, and the reason why we do not see the effect of this difference is that the human eye can only see light in a certain range of speed.

Coincidentally, in the 10 years before Arago's study, infrared and ultraviolet rays were discovered one after another, both of which were invisible to the naked eye. These naturally become the basis of Arago's hypothesis. Now we know that the color of light determines its frequency and does not affect its speed.

So Arago did what he thought was a failed experiment-he didn't observe different speeds of light. Moreover, he gave the experiment a wrong theoretical explanation because he insisted on the theory of particles.

Arago is indeed a brilliant scientist. His research has played a very important role in the development of optics. It was his many experimental studies and discoveries that inspired another French physicist, Fresnel, to establish a complete theory of wave optics. But at the same time, Arago is also a tragic figure, he always seems to lack the courage and vision.

His research was interrupted many times because of the influence of war and political movement. In 1850, when he was finally able to settle down to measure the speed of light in different media, his eyes suddenly went blind. Fortunately, before his death, Fesso and Foucault of France completed the experiment as he suggested, confirming the wave theory that the speed of light is inversely proportional to the refractive index of the medium.

You may be wondering, why is he so stubborn, giving up particles early and saying that hugging etheric waves is not fragrant?

In fact, the etheric fluctuation theory at that time could not explain Arago's experiment either.

According to the etheric wave theory, the speed of light is determined by the refractive index of the ether and the medium, and the motion of the wave source does not affect the speed of light. So different stars, regardless of speed and mass, do not affect the speed of light.

However, according to the law of wave propagation, observers of relative etheric motion will see different speeds of light. It's like when a person runs towards the loudspeaker, the sound he receives becomes faster, while when he is away from the loudspeaker, the sound speed slows down.

Because the earth is orbiting the sun, the earth must move relative to the ether, sometimes close to the stars, sometimes far away from the stars, and the speed of light seen on the earth must change.

This change in the speed of light should lead to different refraction angles. But Arago's experiment did not see different refraction angles!

So it's no wonder Arago didn't fall back so quickly to the etheric wave theory.

However, around 1813, when Arago completed the study of polarized light and the refractive index of transparent media, he began to abandon the particle theory and gradually moved towards the etheric wave theory. When Fresnel first published his paper on the diffraction of light in 1815, Arago made it clear that he supported the wave theory of light.

But as mentioned above, even according to the etheric wave theory, Arago's experiment still cannot be explained! It was not until 1818 that Arago accepted Fresnel's new suggestion that the medium would partially drag the ether.

According to Fresnel, due to the movement of the medium, the ether will be driven, but will not move synchronously with the medium. The degree of its motion is determined by the refractive index of the medium. In this way, the etheric wave theory can explain not only the optical aberration phenomenon, but also Arago's experimental results.

Fresnel's "partial drag etheric" theory seems to be just right. It successfully explains not only the Arago experiment, but also the Eli experiment and the Fesso pipelining experiment. In the nearly 60 years after Fresnel's death, it simply shone brilliantly.

So, what exactly is the "partial drag ether" theory? How does it explain these experiments? Is it really the final theory?

These are all questions worth exploring carefully. As you dig deeper into these questions, you will step closer to the fascinating theory of time and space-Einstein's special theory of relativity.

reference

Https://en.wikipedia.org/wiki/Fran%C3%A7ois_Arago

Https://www.cs.mcgill.ca/~rwest/wikispeedia/wpcd/wp/l/Luminiferous_aether.htm

Https://www.desy.de/user/projects/Physics/Relativity/SpeedOfLight/measure_c.html

Https://www.animations.physics.unsw.edu.au/jw/light/Poisson-Arago-dot.html

Https://doi.org/10.1088/0143-0807/26/1/020

This article is from the official account of Wechat: University Physics (ID:wuliboke), by Xue Debao.

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